In a psychoacoustic audio encoder, coding of low-bitrate stereophonic signals is often achieved by what is referred to as joint-stereo techniques. In its simplest form, instead of transmitting two independent channels, joint-stereo techniques transmit the sum “M” of both channels together with a coefficient “C” that determines the direction in which this signal will be presented at the decoder:Lr=M*sin(C), Rr=M*cos(C)
where Lr and Rr are the left and right channel signals which are reconstructed in-phase with respect to one another. Typically, the audio signal is split into several audio frequency bands and one such coefficient is transmitted per group of frequency bands (e.g. to save bits over transmitting both channels because the coefficient can be heavily quantized). Although joint-stereo techniques may be well-suited for coding of low-bitrate stereophonic signals, they are not particularly well-suited for encoding matrix-surround sound signals as information (such as phase relationships) typically needed by the receiver for matrix-surround sound processing/decoding is not preserved using such joint-stereo techniques. Matrix-surround encoding is essentially an approach to encoding surround sound in which third and sometimes fourth channels of sound are folded into the two front stereo channels and later partially decoded in a reverse operation. The center channel is decoded by using signals common to both left and right channels, whereas the surround channel is decoded by extracting the sounds with inverse waveforms.
As opposed to joint-stereo techniques, dual channel or dual-mono encoding and mid/side coding techniques do tend to preserve information needed for surround sound processing/decoding. Dual channel or dual-mono coding encodes the two input channels (i.e. left and right) as separate entities, whereas in mid/side coding, the mid (L+R) channel having a mono component and the side (L−R) channel having a phase component are encoded separately. Unfortunately however, existing surround sound preserving coding techniques are high bandwidth techniques that are not suitable for transmission over low-bitrate connections.